The present invention generally relates to an electronically controlled fuel injection system for an internal combustion engine. More particularly, the invention is directed to the provision of an electronically controlled fuel injection system which can assure an improved operation performance and optimum air-fuel ratio control by correcting deceleration for compensating delay involved in the air-fuel ratio control based on an air flow sensor in dependence on the opening degree of a throttle valve.
In general, the air flow fed to an internal combustion engine varies in proportion to the opening degree of a throttle valve (also referred to as the throttle aperture). However, in actuality, the air flow can not immediately follow a change in the throttle aperture. For example, when the throttle valve is closed completely starting from the fully opened state, the air flow can vary correspondingly only with a time lag. This can be explained by the fact that the air suction passage extending from the position of the throttle valve has a predetermined length and that the air flow sensor is disposed at a position upstream of the throttle valve. Under the circumstance, the air-fuel ratio control can not be accomplished in a satisfactory manner. More specifically, when a motor vehicle is to be decelerated (through engine braking), the throttle valve is moved in the closing direction, as a result of which the air fuel mixture must become lean. However, in actuality, since the optimum fuel supply injected through the electronically controlled fuel injector is arithmetically determined on the basis of the intake air flow detected by the flow sensor, the air fuel mixture tends to be temporarily enriched, resulting in a condition in which deceleration through engine braking can not take place in a desired manner. To overcome this difficulty, it is known to correct the delay involved in the air/fuel ratio control by opening and/or closing the throttle valve so that the output signal of the air flow sensor can be utilized in the control of the fuel supply without a time lag.
In an electronically controlled fuel injection system disclosed in Japanese Patent Application Laid-Open No. 185949/1983, correction of deceleration is effected by using a so-called throttle sensor. More specifically, when the rate of change or derivative of the output signal exceeds a predetermined value, the amount of fuel supply arithmetically determined on the basis of the amount of intake air detected by the air flow sensor is corrected by multiplying it with a coefficient of a certain value (e.g. 0.9). This correction is referred to as the correction of deceleration while the coefficient is referred to as the deceleration correcting coefficient. The known deceleration correcting system is however disadvantageous in that the correction is made to the same extent for different changes in the throttle aperture. For example, referring to FIG. 1 of the accompanying drawings, when the correction of deceleration is performed by multiplying by the correcting coefficient having a value predetermined for a given rate of change in the throttle aperture or derivative of the throttle sensor output, the same correction will be made for both the cases where the throttle aperture is changed to a level A shown in FIG. 1 and where the throttle aperture is decreased to a lower level B, notwithstanding the fact that a change in the air flow in the first mentioned case differs from the second case, thus making it impossible to realize the optimim airflow ratio control.